Families will be studied who have been identified to have a mutation in the Gene (SCN5A) which is responsible for the cellular changes that produce a long QT. The gene is located on chromosome #3 and has been named LQT3. The aim of this study is to provide new insights into more effective therapy to prevent irregular heart rhythms (arrhythmias) in this particular form of LQTS. This will be done by testing the effectiveness of a sodium channel-specific drug, mexiletine, in shortening the QT interval on the electrocardiogram.